1. Field of the Invention
The present invention relates to a plasma addressing electro-optical device having a dual-layer structure constituted of an electro-optical cell such as a liquid crystal cell and a plasma cell, and more particularly to a structure of the plasma cell.
2. Description of the Related Art
In a matrix type electro-optical device such as a liquid crystal display device employing a liquid crystal cell as an electro-optical cell, an active matrix addressing system is generally known as a means for effecting a high resolution and a high contrast. In such an active matrix addressing system, a plurality of switching elements such as thin film transistors are individually provided for a plurality of picture elements, and the switching elements are driven in a line sequential manner. However, as a plurality of semiconductor elements such as thin film transistors must be provided on a substrate, there occurs a problem such that a yield is reduced particularly in the case of enlarging a picture area of the device.
To solve this problem, there has been proposed in Japanese Patent Laid-open Publication No. 1-217396 (corresponding to U.S. Pat. Nos. 4,896,149 and 5,077,553) a system of utilizing plasma switches rather than the switching elements such as thin film transistors. There will now be described in brief a construction of a plasma addressing electro-optical device having a liquid crystal cell adapted to be driven by utilizing such switches based on plasma discharge. As shown in FIG. 13, the conventional plasma addressing electro-optical device has a laminated flat panel structure constituted of a liquid crystal cell 101, a plasma cell 102 and an intermediate plate 103 interposed between the liquid crystal cell 101 and the plasma cell 102. The plasma cell 102 is formed from a glass substrate 104. A plurality of parallel grooves 105 are formed on an upper surface of the glass substrate 104. The grooves 105 extend in a direction of rows of a matrix, for example. The grooves 105 are enclosed by the intermediate plate 103 to thereby define a plurality of individual plasma chambers 106. An ionizable gas is sealed in each of the plasma chambers 106. A crest portion 107 formed between adjacent ones of the grooves 105 serves as a barrier rib for separating the plasma chambers 106 from each other and as a gap spacer for the plasma chambers 106. A pair of plasma electrodes 108 and 109 extending in parallel to each other are provided on a bottom portion of each groove 105. The pair of plasma electrodes 108 and 109 function as an anode A and a cathode K for ionizing the gas in each plasma chamber 106 to generate a discharge plasma. Such a discharge region between the anode A and the cathode K serves as a line scanning unit. The bottom portion of each groove 105 is formed as a curved surface, and the pair of anode A and cathode K are disposed on the curved surface to incline with respect to each other at a predetermined angle in opposed relationship to each other. Accordingly, plasma discharge easily occurs between an electrode surface of the anode A and an electrode surface of the cathode K.
On the other hand, the liquid crystal cell 101 is constructed of a transparent substrate 110 and a liquid crystal layer 111 formed between the transparent substrate 110 and the intermediate plate 103. The transparent substrate 110 is opposed to the intermediate plate 103 with a predetermined space defined therebetween, and the liquid crystal layer 111 is filled in this space. A plurality of signal electrodes 112 are formed on an inside or lower surface of the transparent substrate 110 so as to extend in perpendicular relationship to the plasma chambers 106, thereby forming column driving units. The signal electrodes 112 are formed of a transparent conductive material. Accordingly, a plurality of picture elements are arranged in a matrix form at the intersections between the column driving units of the liquid crystal cell 101 and the line scanning units of the plasma cell 102.
In the electro-optical device having the above construction, the plasma chambers 106 of the plasma cell 102 are selectively scanned in a line sequential manner, and in synchronism therewith an analog driving voltage is applied to the signal electrodes 112 of the liquid crystal cell 101, thereby effecting display driving. When the plasma discharge occurs in the plasma chambers 106, an anode potential is reached almost over each plasma chamber 106 to select the picture elements at every line. That is, the plasma chambers 106 function as sampling switches. When a driving voltage is applied to each picture element under the condition where the plasma sampling switches are ON, sampling and holding are performed to control an on or off state of each picture element. Even after the plasma sampling switches become OFF, the analog driving voltage is held in each picture element as it is.
The above electro-optical device utilizing the discharge plasma may have an advantage such that a picture area of the device can be enlarged more easily than that in an electro-optical device using semiconductor elements. However, various problems remain in practical application. For example, the formation of the grooves 105 defining the plasma chambers 106 on the substrate 104 such as a glass substrate is considerably difficult in manufacturing. In particular, it is very difficult to form the grooves 105 with a high density. Further, while the plasma electrodes 108 and 109 for discharging are necessarily formed in each groove 105, an etching process for forming the plasma electrodes 108 and 109 is troublesome.
In view of the above problems in the related art, the present assignee has already proposed in Japanese Patent Application No. 3-47784 a plasma addressing electro-optical device which can be simply manufactured and is suitable for a large picture area and a high resolution. There will now be described in brief a structure of such an improved device proposed by the present assignee, for the purpose of understanding of the present invention. Referring to FIG. 14, this improved device also has a laminated flat panel structure constituted of a liquid crystal cell 201 and a plasma cell 202. The liquid crystal cell 201 has basically the same structure as that of the liquid crystal cell 101 shown in FIG. 13. On the other hand, the plasma cell 202 is constituted of an intermediate plate 203 and a lower substrate 204 opposed to each other with a predetermined space defined therebetween. This space is partitioned by a plurality of barrier ribs 206 to define a plurality of plasma chambers 205. Each plasma chamber 205 forms a discharge region as a line scanning unit. An ionizable gas is sealed in each plasma chamber 205. The barrier ribs 206 are formed by a printing process. A pair of plasma electrodes 207 are formed in each separate discharge region to serve as an anode A and a cathode K. The plasma electrodes 207 are formed on an upper flat surface of the substrate 204 by a printing process. As the printing process is a very simple technique, the productivity and the workability can be improved as compared with those in forming the grooves in the related art. Further, the printing process is for enlargement of a picture area.
Various samples having the flat panel structure shown in FIG. 14 were actually prepared to carry out a plasma discharge test. As the test result, some of the samples showed a stable and uniform discharge between the anode and the cathode, but some of the samples showed almost no discharge however a sealed gas pressure and an applied voltage were set. This latter phenomenon was initially considered to be due to any defect in a production process, such as contamination in the plasma cell. However, there was almost no difference in this phenomenon even by changing the forming method for the electrodes or the kind of the electrodes. In subsequent investigation, it has been concluded that this discharge defect is caused by the arrangement itself of the electrodes. That is, the plasma electrodes shown in FIG. 14 are formed from a conductive thick film or the like, and the plasma cell is of a surface discharge type such that plasma discharge occurs between the upper surface of the anode and the upper surface of the cathode. However, since the anode and the cathode are formed on the flat surface of the substrate, the upper surfaces of the anode and the cathode are of the same level, and they are not opposed to each other. In contrast, the upper surfaces of the anode and the cathode shown in FIG. 13 are opposed to each other at a given inclined angle. In this manner, the electrode surfaces of the plasma cell shown in FIG. 14 are not opposed to each other in spite of the surface discharge type, thus possibly causing the above discharge defect.
In other words, the above test has taught that a structural limitation or condition is required to realize stable plasma discharge in the plasma cell structure shown in FIG. 14.